Open-top vehicles are commonly used to transport bulk products such as grain and sand because the opening enables the bulk product to be quickly deposited into the vehicle from above. A drawback of the open top is that the product may be blown or jostled out of the open top of the vehicle as it moves down the road. The lost material not only reduces the amount of the load ultimately delivered to its final destination, but also may hit or otherwise damage other vehicles or persons on the road.
Another drawback of open top vehicles is that the product is exposed to the ambient environment such as rain, hail, snow, high winds and direct sunlight. Those factors may damage the product. For example, bulk grain is vulnerable to rain and snow.
Canvas and vinyl covers were developed to cover the open top. Early systems incorporated hardware that rolled the cover between an open configuration on one side of the top of the truck box or trailer and a closed configuration in which the cover was spread over the top and secured in position. Examples of manually operated cover systems are disclosed in Sibley, U.S. Pat. No. 3,546,197; Sibley, U.S. Pat. No. 3,549,199; Sibley, U.S. Pat. No. 3,628,826; Bachand et al., U.S. Pat. No. 3,868,142; Peteratti, U.S. Pat. No. 4,082,347; Peteratti, U.S. Pat. No. 4,126,351; Dimmer U.S. Pat. No. Re. 31,746, and Compton, U.S. Pat. No. 4,516,802.
In the Dimmer prior art patent, a crank shaft is connected to the roll tube (that carries the cover) by a Universal joint. The U-joint allows the crank shaft to be used in two ways. First, it is used for rolling and unrolling, by pulling the shaft away from the trailer, and rotating it. Because the U-joint is unlocked when the shaft is moved away from the trailer, the rotational movement of the crank is transmitted to rotate the tube. Rotation of the roll tube unfurls the material on the roll shaft across the open top to cover it.
The second use of the U-joint is to lock the roll tube in place. When the rolling or unrolling described above is completed, the operator brings the shaft the back toward the trailer to a more perpendicular orientation relative to the roll tube. In this position, the U-joint is locked and further rotational movement of both the crank and the tube is prevented. This is the configuration when the truck box or trailer is moved on the highway.
More recently, motorized cover systems were developed. Such systems eliminated the crank shaft and U-joint, and attached an electric motor to the end of the roll shaft where the U-joint was previously located. An arm supported the motor in place at the height of the top of the truck box or trailer as it rolled the roll shaft from one side of the trailer to another. The arm also secured the motor against counter rotational forces resulting from the torque applied by the motor to the roll shaft. The other end of the arm was pivotally attached to the trailer to allow the arm to sweep across the front of the trailer from open to closed position, or in the reverse direction.
A motorized cover system is shown in Schmeichel, U.S. Pat. No. 7,188,887 (see also U.S. Pat. No. 7,195,304). It shows a cover system with motor attached to the end of a roll shaft to roll the shaft from an open or unrolled position to a closed configuration. The motor is mounted on an arm assembly that includes two arm sections that are flexibly connected to each other.
In typical prior art motorized cover systems, the motor is welded or otherwise rigidly attached to the arm, as illustrated in FIG. 2 of the Schmeichel patent. The rigidity of the attachment transferred the torque of the motor to the roller or roll shaft. Some movement of the motor in a generally upward direction was allowed by the flexible connection between the two arm sections. This accommodated the variance in distance between the pivot point of the arm attachment to the truck and the top of the trailer (or heap load extending above the top of the trailer). However, because the motor was rigidly connected both to the arm and the roll shaft, the second arm section-shaft angle never varied.
This operation is not problematic if all the components are truly square and perpendicular (or close to perpendicular) both when manufactured and during the life span of the cover system. In reality, trailers not fabricated to perfect specifications, and arms and roll tubes may be bent during installation. Further departure from optimum angles occurs during use over time as a result of collusions with loading machinery, other vehicles, or materials on their way into or out of the box or trailer.
An example of a departure from desired angles and specifications occurs when the roll tube or shaft becomes bent. In this situation, the angle of the axis of the portion of the roll shaft attached to the motor varies from a right angle with the arm. If the axis of the end of the shaft angles downward relative to the axis of a perfectly straight (not bent) shaft, there is a force applied against the arm-shaft connection toward expanding the angle to more than 90 degrees. On the other hand, if the shaft angles upward, there is a force applied against the arm-shaft connection toward reducing the angle to less than 90 degrees.
In the bent shaft situation, the rolling and unrolling operations continuously varies the angle of the force on the arm-shaft connection. During a single rotation, there will be substantial force to try to narrow the arm-shaft angle to less than 90 degrees against the rigid connection, as well as substantial force trying to widen the angle. (It will also twist the second arm section to the right and the left.) The center line of the drive shaft of the motor (or transmission) attempts to follow the axis of the roll tube and twist about both the vertical and horizontal axes, transferring this force to the weld or other rigid attachment between the motor or motor mount and the arm.
During the entire opening or closing operation, there are many rotations, creating an oscillating stress at this attachment, down the arm and across the length of the shaft. Over time, this creates excessive wear and premature deterioration of the cover and metal parts, and potentially the catastrophic failure of the arm-motor connection.